Tag: Black hole

A black hole – hitherto an invisible celestial body – was in cosmological vocabulary even before Einstein’s theory of relativity in 1915. But when the relativity theory predicted with full scientific rigour that a massive stellar body can have such a strong gravitational pull that nothing, no object, not even electromagnetic radiation such as light, can escape from it, the concept of a black hole became firmly established in scientific parlance. But it remained at that time only a mathematical curiosity, as no scientific evidence or mechanism of formation of a black hole was put forward. However, it became a realistic possibility after the detection of pulsars some decades later.

The detection of pulsars (rotating neutron stars) by Jocelyn Bell Burnell, a research student at the University of Cambridge in 1967, gave renewed spurt to the concept of gravitational collapse and the formation of black holes. A normal star, when it comes to the end of its life due to lack of fusion fuel, collapses under its own gravity and becomes a neutron star. It may be mentioned that an atom consists of neutrons (neutral in charge) and positively charged protons and negatively charged electrons. If gravity becomes too strong, protons and electrons are pulled together to merge with each other, neutralise their charges and become neutrons and the whole star becomes a neutron star. (For the detection of neutron star, which was considered as “one of the most significant scientific achievements of the 20th century” by the Nobel Committee, her supervisor and another astronomer were awarded Nobel prize in Physics in 1974, but Jocelyn Bell was not even mentioned in the citation. However, years later, in 2018, she was awarded the Special Breakthrough Prize in Fundamental Physics. She donated the whole of the £2.3 million prize money to the Institute of Physics in the UK to help female, minority, and refugee students become physics researchers.

Not all stars eventually become neutron stars. If the mass of a
star is less than 2.6 times the mass of the Sun, the gravity would not be
strong enough to turn it into a neutron star. The gravitational pull in a
neutron star ultimately becomes so strong that all its mass and its nearby matters
are pulled to a small volume and the star becomes a black hole. A black hole
can merge with another black hole to become a bigger and stronger black hole.

It is speculated that there are black holes of various sizes in
most of the galaxies and in some galaxies, there are supermassive black holes
at their centres. The nearest black hole from Earth is quite a few thousand
light-years away; but they exert no influence on this planet. The supermassive
black hole in our galaxy (the Milky Way) is about 26,000 light-years away.

Despite the name, a black hole is not all black. The gas and dust trapped around the edges of the black hole are compacted so densely and heated up so enormously that there are literally gigantic cauldrons of fire around the periphery of a black hole. The temperatures can be around billions of degrees!

The first direct visual evidence of a black hole had been produced on 10 April 2019 by a team of over 200 international experts working in a number of countries. The Event Horizon Telescope (EHT) was used to detect the existence of a colossal black hole in M87 galaxy, in the Virgo galaxy cluster. The computer simulation from data collected in the EHT is shown below. This black hole is located some 55 million light-years from the Earth and its estimated mass is 6.5 billion times that of the Sun! So, this black hole is truly a monster of a black hole.

Computer simulation of black hole from real data

Although it is a monstrous black hole, its size is quite small
and it is enormously far away (520 million million million kilometres away) from
Earth. To observe directly that elusive black body that far away, astronomers
require a telescope with an angular resolution so sharp that it would be like
spotting an apple on the surface of Moon from Earth and the aerial dish that
would be required for such a detection would be around the size of Earth!
Obviously, that is not possible.

Instead, the international team of experts devised a Very Long Baseline Interferometry (VLBI) technique, which involves picking up radio signals (wavelength 1.3 mm) by a network of radio telescopes scattered around the globe. The locations of these eight radio-telescopes are shown below. When radio signals from these radio-telescopes are joined up, taking into account their geographical locations, lapsed times for signal detection etc, and processed in a supercomputer, an image can gradually be built up of the bright part of the periphery of the black hole.

Locations of Event Horizon Telescopes (EHT)

The
key feature of a black hole is its event horizon – the boundary at which even
light cannot escape its gravitational pull. The size of the event horizon
depends on the mass of the black hole. Once an object crosses the boundary of
the event horizon, there is absolutely no chance of coming back. A lead
astronomer from MIT working on this EHT team said, “Black hole is a one-way
door out of this universe.”

The
general theory of relativity also predicted that a black hole will have a
“shadow” around it, which may be around three times larger than the event
horizon size. This shadow is caused by gravitational bending of light by the
black hole. If something gets nearer the shadow, it can possibly escape the
gravitational pull of the black hole, if its speed is sufficiently high
(comparable to the speed of light).

It
is postulated that the “shadow” comprises a number of rings around the event
horizon. The nearer a ring is to the event horizon, the more rigorous and compact
it is with extreme pressure-temperature conditions.

If, hypothetically, an unfortunate human being falls even into the outer ring of a “shadow”, he will be pulled towards the black hole initially slowly and then progressively strongly – his leg will be pulled more vigorously than his upper part and consequently, his body will be deformed into a long thin strip like a spaghetti. And when that spaghetti shape crosses the event horizon, it will be stretched so much that it will become a very thin and very long string of atoms!

Is wormhole the link between a black hole and a white hole?

The
general perception of a black hole is that it is a monster vacuum cleaner where
everything, even light, is sucked into it through a funnel and nothing,
absolutely nothing, can come out. It absorbs enormous amount of matter and
squashes them into tiny volumes. What happens to this gigantic amount of matter
is a mystery, a black mystery.

There
are two parallel streams of pure speculative thoughts. One is that when a black
hole becomes too big – either by incessantly swallowing up matters from its
surroundings or by merger with other black holes – a super-giant explosion,
more like a big bang, may take place. So, a black hole may be the mother of a new
big bang, a new generation of universe.

The other thought is that the funnel of a black hole is connected through a neck, called the wormhole, to a different spacetime and hence a different universe at the other end. All the materials that a black hole sucks up at the front end in this universe go through the wormhole to another reverse funnel where all the materials are spewed out into a different spacetime. That funnel is called the white hole. Thus, a black hole and a white hole is a conjugate pair – a connection between two universes! But the question is, since there are billions of black holes in our universe, then there could be billions of corresponding wormholes and white holes and universes.

One universe is big enough or bad enough for human minds to contemplate, billions of universes will make humans go crazy.

In 1974, by predicting the apparently paradoxical concept of radiation emanating from black holes, Hawking reminded us that mass and energy are two sides of the same coin.

We humans are a recent phenomenon in the Universe that is very old, mostly imperceptible and beyond our comprehension. Had it not been for great scientists like Isaac Newton, Albert Einstein, Edwin Hubble, Karl Schwarzschild, Subrahmanyan Chandrasekhar, Stephen Hawking and many more who unlocked the enduring mysteries of the boundless Universe, it would have been a struggle for lesser mortals like us getting our bearings straightened about our place in the cosmos. Their ground-breaking work, forever, changed our view of the “heavens.”

Postulated in 1687, Newton’s law of gravity was a beautiful synthesis between terrestrial and celestial phenomenon, reaching across the vast expanse of the Universe. It allows us to study the waltzing motion of the planets, moons, stars and other objects in the sky with clockwork precision.

Einstein’s special relativity, published in 1905, tells us that time is not only elastic, it is also the fourth component of the spacetime fabric of the Universe. Ten years later, his general relativity redefined gravity as matter’s response to the curving of spacetime caused by surrounding massive objects.

In 1916, Schwarzschild found that the solution of Einstein’s general relativity equations characterized something that confounds common sense ‒ an unfathomable hole drilled in the superstructure of the Universe. Today, we call this voracious gravitational sinkhole a black hole, a single point of zero volume and infinite density.

Much to Einstein’s consternation, in 1929, Hubble discovered that the Universe is expanding in size. His “constant” enabled us to estimate the age of the Universe. In 1930, Chandrasekhar’s calculations indicated that when a massive star runs out of fuel, it would blow itself apart in a spectacular but violent explosion and then collapse into a black hole.

Black holes were discovered in 1971, when astronomers detected a hint of radio wave emissions coming from an object in the constellation Cygnus. The emissions were later interpreted as the fingerprint of the black hole Cygnus X-1. Since then, numerous black holes, including supermassive ones, have been detected in our own Galaxy ‒ The Milky Way, and elsewhere in the Universe. According to NASA, supermassive black holes are growing faster than the rate at which stars are being formed in their galaxies.

Cloaked behind the event horizon, which is not a physical barrier but just an information barrier, it must seem that there is no way of getting mass from the black hole back out into outer space. No way, that is, not until the British physicist Stephen Hawking, arguably one of the greatest minds in scientific history, joined the Big League of Cosmology in the mid-twentieth century. With his seminal contributions to the fields of astrophysics, general relativity, quantum gravity and black holes, he raised the field of cosmology from a niche topic to a well-developed subject in the forefront of science.

In 1974, by predicting the apparently paradoxical concept of radiation emanating from black holes, Hawking reminded us that mass and energy are two sides of the same coin. He was able to show that a black hole, like any other body whose temperature is not absolute zero, emits energy in the form of radiation, energy now known as Hawking Radiation.

The continual emission of radiation causes the black hole to shrink in mass. In other words, black holes “evaporate,” although the time it takes for a solar-mass black hole to evaporate completely is immensely long ‒ vastly larger than the age of the Universe, which is 13.7 billion years. The implications are nonetheless important ‒ even black holes evolve and die.

One of the Gordian knots of cosmology is the missing mass of the Universe. There is irrefutable evidence that visible matter accounts for only four percent of the Universe’s mass. The remaining 96 percent is invisible of which 73 percent is attributed to a pervasive “dark energy,” believed to be manifestation of an extremely powerful repulsive force that is causing the expansion of the Universe to accelerate. The additional 23 percent is thought to be dark matter whose origin obviously is the many black holes spread throughout the cosmos. However, their total mass does not add up to account for all the dark matter.

To address this issue, in 1971, Hawking advanced the idea that in the intergalactic space, there may be “mini” black holes with very small masses ‒ much smaller than the mass of the Earth ‒ yet numerous enough to account for most of the unaccounted dark matter. He hypothesized that they may have been formed during the first instants of chaos following the Big Bang when matter existed in a hot, soupy plasma. Since mini black holes have not been detected so far, Hawking lamented: “This is a pity, because if they had, I would have got a Nobel Prize.”

During the 1980s, Hawking devoted much of his time contributing to the theory of cosmic inflation ‒ the expansion of the Universe at an exponential pace before settling down to expand at a slower pace. In particular, he demonstrated how minuscule variations in the distribution of matter during this period of expansion, known as the Planck era, helped shape the spread of galaxies in the Universe.

As noted above, the core remnant of a high-mass star would eventually collapse all the way to a point ‒ a so-called singularity. Having said that, singularities are places where laws of physics break down. Consequently, some very strange things may occur near them. As suggested by Hawking, these strange things could be, for instance, gateway to other universes, or time travel, but none has been proved, and certainly none has been observed. These suggestions cause serious problems for many of our cherished laws of physics, including causality ‒ the idea that cause should precede the effect, which runs into immediate problem if time travel is possible ‒ and energy conservation, which is violated if matter can hop from one universe to another through a black hole.

While scientists know Hawking for his work on cosmology, millions of others know him because of his book “A Brief History of Time.” His lucid explanation of the mechanism leading to the creation of the Universe, our place in it, how we got there, where did space and time come from, and where we might be going made the notoriously difficult subject of cosmology more understandable to the layperson.
In a follow-up book titled “The Grand Design,” Hawking outlines his consuming quest for the long-dreamed-of “Theory of Everything,” the quantum theory of gravity. Such a theory would unify the two pillars of twentieth century physics, general relativity and quantum theory.

Known as the M-Theory (M stands for Mother-of-All), it would enable us to understand all phenomena in space-time, especially the first split second of cosmic creation, when everything was unimaginably small and densely packed.

Hawking was about as pure an atheist as one can be. He dismissed the existence of an omnipotent by noting that “regularities in the motion of astronomical bodies such as the sun, the moon, and the planets suggested that they were governed by fixed laws rather than being subjected to the arbitrary whims of gods and demons.” Nevertheless, in December 2016, he had a surprising but cordial encounter with Pope Francis at a convention on Big Bang in Rome.

Besides being a genius, Hawking’s celebrity status derives from his spunk in the face of physical adversity. Born on 8 January 1942 in Oxford, England, Hawking was diagnosed with a debilitating, incurable neuromuscular disorder, commonly known as motor neurone disease (MND), when he was just 21 years old. Although doctors predicted that he has only two more years to live, he lived another 55 years and died on 14 March, 2018. There are some interesting anecdotal coincidences. Hawking was born on the 300th anniversary of Galileo’s death and died on the 139th anniversary of Einstein’s birth. Following his cremation, his ashes will be interred on 15th June 2018 in Westminster Abbey’s nave, next to the grave of Isaac Newton and close to Charles Darwin.

Instead of ruing about his mortality, Hawking considered his illness as a blessing, allowing him, in his own words, “to focus more resolutely on what he could do with his life.” Indeed, with a crumpled, voiceless body ensconced in a wheelchair, he soared and established an exalted scientific reputation as the most recognizable scientist of the modern era. The name of this supernova of cosmology will be engraved in the sands of time as long as humanity lives.

The writer, Quamrul Haider, is a Professor of Physics at Fordham University, New York

Around 1020 BC, a shepherd boy named David took on the mighty Goliath and felled him with just a pebble and a sling on a battlefield in ancient Palestine. Since then, the names of David and Goliath signify battles between underdogs and giants. Now fast forward to early 20th century. The David of the scientific world is an Indian child prodigy named Subrahmanyan Chandrasekhar, an outstanding astrophysicist and a towering figure of 20th century science, who published his first scientific paper in the Proceedings of the Royal Society of London when he was just 19 years old.

Born in Lahore on October 19, 1910, Chandrasekhar studied physics at the Presidency College in Madras (now Chennai). He obtained his BSc degree in 1930, the year his paternal uncle CV Raman became the first Indian to win the Nobel Prize in Physics. Due to his stellar academic achievements, Chandrasekhar was awarded a scholarship to pursue doctoral studies at Trinity College in Cambridge, UK. Accordingly, he set sail for London in July 1930. He earned the doctorate degree in 1933.

During the long voyage to London, 19-year-old Chandrasekhar had enough time to pore over a problem that had bothered him for a long time – what happens to stars in the terminal stage of their life. On board the ship, he completed the calculations showing that the fate of a star depends on a critical mass, which is 1.4 times the solar mass.

Now known as the ‘Chandrasekhar Limit’, it is the limiting mass of white dwarfs – the end-stage of Earth-sized stars, but about 200,000 times as dense. If a star’s mass falls below the limit, it would end up in the stellar graveyard as a white dwarf. Otherwise, it would blow itself apart in a spectacular but violent supernova explosion and then collapse into a smaller – about 20 km in diameter – remnant called a neutron star, or possibly into a single massive point with no dimensions and infinite density. Indeed, this was the first prediction of what we now call a black hole – an entity from which nothing can escape, not even light.

Unfortunately, Chandrasekhar’s view was obstinately opposed by Arthur Eddington, the Goliath of astrophysics of the era, who knew about the possibility of black holes but refused to believe they could exist. And, thus, began the fight between David and Goliath of the scientific world. Eddington found Chandrasekhar’s conclusion about the fate of the stars unacceptable and launched an attack on his work, both publicly and privately.

On January 11, 1935, after Chandrasekhar presented the results of his research at a meeting of the Royal Astronomical Society in London, Eddington ridiculed the Chandrasekhar Limit as a “reductio ad absurdum”, meaning a logically absurd conclusion. He steadfastly refused to consider the idea that stars might collapse to nothing. He trashed Chandrasekhar’s theory as mere mathematical gimmick with no basis in reality.

Eddington’s arrogance and criticism devastated Chandrasekhar. He was shocked that instead of giving him credit for solving a challenging problem, Eddington was bent on destroying his work. But Chandrasekhar held his ground. In his fight to counter Eddington, he was assured by Niels Bohr, the 1922 Physics Nobel Laureate, that Eddington was patently wrong and should be ignored.

Nevertheless, the 1935 incident led Chandrasekhar to believe that an influential figure like Eddington could derail his career if he stays in Europe. He, therefore, moved to Chicago in 1937, where the University of Chicago provided him with an intellectual home – first at the Yerkes Observatory in Wisconsin and then at the physics department in the city campus, where he stayed until his death on August 21, 1995.

Two years after he moved to Chicago, Chandrasekhar and Eddington had their final squaring off in Paris. Undeterred in his conviction that there must be a law of nature “to prevent a star from behaving in this absurd way,” Eddington claimed that there was no experimental test that could lend support to Chandrasekhar’s theory. Nonetheless, he apologised to Chandrasekhar for questioning his calculation. “I am sorry if I hurt you,” Eddington said. When Chandrasekhar asked Eddington whether he had changed his mind, he retorted, “No.” Chandrasekhar then replied, “What are you sorry about then?” and walked away.

Although late in life Chandrasekhar and Eddington exchanged some cordial letters, they never discussed the issues concerning the fate of the stars. He eventually made peace with Eddington, who promoted his election to the Royal Society in 1944. Eddington died on November 22, 1944.

The eulogy Chandrasekhar gave for Eddington at the University of Chicago says it all about his graciousness and magnanimity. “I believe that anyone who has known Eddington will agree that he was a man of the highest integrity and character. I do not believe for example, that he ever thought harshly of anyone,” he said.

Thirty-one years after the infamous encounter with Eddington, physicists finally acknowledged the relevance and importance of the Chandrasekhar Limit. Moreover, in 1971, the first black hole was discovered. And as a tribute to Chandrasekhar’s contribution to astrophysics, NASA named one of its space-based observatories after him – the Chandra X-ray Observatory, specially designed to detect stars spiralling into black holes. Since its launch on July 23, 1999, this flagship observatory of NASA has not only discovered numerous black holes, quasars and supernovas, but also allowed us to look at a side of the cosmos that is invisible to the human eye.

Chandrasekhar’s ultimate vindication was the Nobel Prize in Physics awarded to him in 1983 for his ground-breaking work on the structure and evolution of stars. In 1984, he received the Royal Society’s highest award, the Copley Medal.

The writer, Quamrul Haider, is a Professor of Physics at Fordham University, New York.

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